A direct current arc furnace requires conduction of the electric arcing power through the melt in the hearth of the furnace, in the usual arrangement where the melt has one polarity and the arcing electrode or electrodes have the other polarity. To make the hearth itself electrically conductive for this purpose, is generally considered to be unsatisfactory for various reasons.
Therefore, hearth electrodes of the liquid-solid type have been suggested by the prior art as exemplified by U.S. Pat. No. 3,789,127, dated Jan. 29, 1974, and in a much improved form, by the U.S. Stenkvist application Ser. No. 587,714, filed June 17, 1975.
The above type is exemplified by a steel billet extending through a furnace construction and having an inside end positioned for contact by a steel melt in the furnace and an outside end provided with cooling means. In operation, the inside end portion melts to the steel's liquid phase, but the steel billet remaining solid from its outer end and safely inwardly towards it inside end, by being cooled, as by watercooling, the solid outside end being available to form an electrical connection with a direct current power source which is, of course, also connected to arcing electrode or electrodes.
To accommodate such a hearth electrode, the furnace vessel is constructed with a suitably located passage for insertion of the billet which, as disclosed by the previously stated patent application, is preferably of a curved shape permitting the outer end which remains solid, to be positioned at a height safely above the melt level in the furnace.
With the billet positioned in the furnace vessel opening, refractory material in a generally plastic or moldable state, is rammed into the opening around the billet throughout the length of the billet which must extend through the furnace hearth or wall.
The refractory material may, for example, be a magnesite compound made in a plastic or moldable condition by being mixed with water. Therefore, the refractory used to fill the space around the electrode or billet formed by the furnace construction opening, which for practical reasons has a cross sectional area substantially larger than does the billet, is moist in the case of any new electrode installation and requires thorough drying before the furnace can be put into operation.
Heretofore, to dry the rammed and moist refractory around the billet in a new installation, it has been customary to heat the inside end of the billet, not required for an electrical connection with a power line, so that by conduction throughout the billet the necessary drying process is hopefully accelerated. However, steel has a relatively low thermal conductivity, so the conduction of heat throughout its length is relatively poor, making the drying undesirably slow.